1. Field of the Invention
The present invention is related to resistance measuring devices and, more particularly, is directed towards a novel structure designed for ease of measurement of the resistance of a seam in a shelter used for electromagnetic shielding purposes.
2. Description of the Prior Art
Army shelters are built with walls made from metal or metal covered composites for shielding, and support. The shielding is designed to assure the electromagnetic and nuclear survivability of critical command, control and communications electronic warfare sheltered systems. These metal walls are usually joined either by bolts or welding. The resulting seams between the metal walls have a measurable resistance. Seam resistance is important in that it is a measure of the shielding effectiveness of the shelter. If the shelter is subject to high electric and magnetic fields, currents will be developed on the outside walls and those currents may encounter a seam. If the seam has a high resistance (due to damage, oxidation, age degradation or uneven torque on the seam bolts), a large voltage will be developed across the seam and energy will be radiated into the shelter. If the shelter contains electronic equipment, the radiated energy could couple into the electronic equipment and cause considerable damage.
The coupling through seams is reasonably well understood theoretically. The open circuit voltage, V.sub.s (t) developed across the seam is related to the surface current density, J.sub.s (t), flowing normal to the seam by the time domain expression: ##EQU1## where R.sub.s and L.sub.s are the transfer resistance and inductance in units of ohms-m and henries-m, respectively. In the frequency domain, the equation becomes: EQU V.sub.s (.omega.)=Z.sub.T (.omega.)J.sub.s (.omega.)
where Z.sub.T (.omega.) is commonly referred to as the seam transfer impedance.
The magnetic polarizability per unit length, m, is related to the seam inductance, L.sub.s , by EQU L.sub.s =.mu..sub.o m
where .mu..sub.o is the permeability of free space.
Using the transfer impedance concept, the electric field, E.sub.s, developed across the seam is given by ##EQU2## where a is the width of the seam. This is then the electric field at the seam which is needed to calculate the electromagnetic field in the half-space behind the infinite conducting plane having the seam. Thus, the development of the seam theory assumes that there is no cavity of finite size behind the seam.
In the prior art, the method used for determining seam resistance was cumbersome. Two probes for injecting a current surface density J.sub.s were attached to the wall on either side of the seam. Two additional probes for measuring the open circuit voltage V.sub.s developed across the seam were also attached to either side of the seam. These probes were then moved up and down the length of the seam in order to characterize the resistance of the entire seam. However, because the position of the probes on either side of the seam would vary somewhat in relation to the seam and each other at each reading, error was introduced into the overall characterization. Additional error was introduced because of the varying pressure on the current probes as they were held to the wall only by tape. Also, because the relative distance between the current probes and the voltage probes did not remain constant, further error was encountered. The prior art method also required several operators to conduct these measurements.
It therefore may be appreciated that there is a great need for a portable seam resistance measuring apparatus that will facilitate repeatable measurements and be operable by one individual.